Recently, electric double layer capacitors whose internal resistances are reduced and whose capacities are enlarged have been developed. The electric double layer capacitors are such that it has been expected to apply them to power usage. The electric double layer capacitors are such that it is the principle that a pair of polarizable electrodes are disposed so as to face to each other in an electrolyte solution with a separator interposed therebetween to make a positive electrode and a negative electrode so that electric charges are accumulated in electric double layers, which are formed in the interfaces between the electrodes (polarizable electrodes) and the electrolyte solution.
The static capacity of electric double layer capacitor is believed to be in proportion to the area of polarizable electrode substantially. Hence, as an electrode material for polarizable electrode, activated carbon, which has a large specific surface area, has been used.
Activated carbon has been produced by means of activating treatment (activation treatment), and a large number of functional groups remain on the surface. When adapting such activated carbon to be the electrodes of electric double layer capacitor; using an organic solvent-based electrolyte solution; and applying a voltage between the facing electrodes, the residual functional groups, especially, the hetero element-containing functional groups, which are left on the surface of the carbon electrodes, have reacted with the electrolyte solution to generate gases, such as CO2, CO and H2O, or to form electrically non-conductive film, and thereby such drawbacks have arisen that the internal resistance of electric double layer capacitor enlarges to result in malfunction, or to shorten the longevity of electric double layer capacitor.
As a method for removing such residual functional groups, methods for heating them in an inert gas atmosphere or in a reducing gas atmosphere have been available. However, in these methods, the reactivity to the residual functional groups is so low that it has not been possible to remove the residual functional groups fully.
With respect to such a problem, performing a heat treatment to them together with a transition metal or its compound in a reducing atmosphere is disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 2002-362,912. However, the method using a transition metal or its compound is such that there has been such a problem that it takes a lot of time to carry out the process for removing the transition metal or its compound from the mixture of the activated carbon and the transition metal or its compound so that it hinders the industrialization. Specifically, in this publication, when using a ferromagnetic transition metal, such as Fe, Ni and Co, it is used as a powder having a fine particle diameter, and the removal after the heat treatment is removed by means of magnetic force. When being a transition metal, such as Cu, that is not ferromagnetic, a process is specified in which it is turned into a fine-net shape or a cotton-like shape to use; when being a compound of the aforementioned transition metal, it is dissolved into a solvent, is permeated through a carbonaceous material, and is removed by eluting it out thereof using an acid or base after the heat treatment. Moreover, capacitors, which use the carbonaceous material produced by the method set forth in this publication, are such that there has been a problem in that the charge/discharge characteristics are not sufficient.